Piezoelectric actuator

ABSTRACT

The invention relates to a piezoelectric actuator, in particular for actuating control valves or injection valves in internal combustion engines in motor vehicles, having a piezoelectric actuator body, in particular in the form of a multilayer laminate of layered plies of piezoelectric material and intervening metal or electrically conductive layers acting as electrodes, in which one of the face ends of the actuator body is fixed on an actuator base, and the actuator body is surrounded by a module wall so as to define an interstice there between, and the interstice is filled with an elastic or plastic electrically insulating material of good thermal conductivity, which is solid at least in the temperature range below the operating temperature of the actuator.

PRIOR ART BACKGROUND OF THE INVENTION

The invention is directed to improvements in a piezoelectric actuator,in particular for actuating control valves or injection valves ininternal combustion engines in motor vehicles, having a piezoelectricactuator body, in particular in the form of a multilayer laminate oflayered plies of piezoelectric material and intervening metal orelectrically conductive layers acting as electrodes, these electrodelayers being contacted by electrically conductive common electrodeleads, and one of the face ends of the actuator body is fixed on anactuator base, and the actuator body is surrounded by a module wallwhile maintaining an interstice between them.

One such piezoelectric actuator is disclosed in German Patent DisclosureDE 196 50 900 A1 of Robert Bosch GmbH.

As is well known, piezoelectric actuators can for instance be used forinjection valves of a vehicle motor and in brake systems with anti-lockand traction control systems.

Such injection valves equipped with piezoelectric actuators have aninjection nozzle controlled by a tappetlike closure device. An operativeface toward the nozzle is disposed on the tappet and is acted upon bythe pressure of the fuel supplied to the nozzle; the pressure forcesseek to urge the tappet in the opening direction of the closure device.The tappet protrudes with a plungerlike end, whose cross section islarger than the aforementioned operative face, into a control chamber.The pressure effective there seeks to urge the tappet in the closingdirection of the closure device. The control chamber communicates withthe fuel supply, which is at a high pressure, via an inlet throttle andwith a fuel return line that has only low pressure, via an outlet valvethat is throttled as a rule or is combined with an outlet throttle. Whenthe outlet valve is closed, a high pressure prevails in the controlchamber, by which the tappet is moved in the closing direction of theclosure device, counter to the pressure on its operative face toward thenozzle, or is kept in the closing position. Upon opening of the outletvalve, the pressure in the control chamber drops; the magnitude of thedrop in pressure is determined by the size of the inlet throttle and bythe throttle resistance of the opened outlet valve, or the outletthrottle combined with it. As a result, the pressure in the controlchamber decreases when the outlet valve is opened, in such a way thatthe tappet is moved in the opening direction of the closure device, orheld in the open position, by the pressure forces that are operative onits operative face toward the nozzle.

In comparison with electromagnetically actuated injection valves,piezoelectric actuators can switch faster. However, in the design of apiezoelectric actuator, it must be noted that internal losses in thepiezoelectric body of the actuator cause lost heat, which has to bedissipated so that the actuator will not overheat. Since the ceramicmaterials of the piezoelectric ceramic have poor heat conductivity, thedissipation inside the actuator body, which substantially comprisesceramic material, is unfavorable.

The heat generated by the actuator backs up in the interstice betweenthe actuator body and the module wall, if only air is present in theinterstice.

Cooling the actuator with a liquid coolant, such as fuel, water, motoroil and the like, which is theoretically possible, is unfavorable, firstbecause of the risk of a short circuit from the water component that iscontained both in the fuel and in motor oil, and second because theactuator module is more expensive because of complicated seals, topreclude leakage of the coolant used, especially when the actuatorbecomes heated.

OBJECT OF THE INVENTION

It is therefore the object of the invention to make a genericpiezoelectric actuator possible in such a way that cooling duringoperation is possible without a liquid coolant such as motor oil, wateror fuel; that the piezoelectric actuator can be installed simply, andthat no special seals as in liquid cooling are necessary.

SUMMARY OF THE INVENTION

A piezoelectric actuator according to the invention thus has theadvantage that the heated actuator body can be cooled without fuel,motor oil or water, that is, without the risk of causing a short circuitbecause of the water component, and also without the risk of leakage.

According to the invention, to that end the interstice between theactuator body and the module wall is filled with an elastic or plasticelectrically insulating material with good heat conductivity, which atleast in the temperature range below the operating temperature of theactuator is solid. With this kind of material as a cooling medium, therisk of leakage disappears.

Cooling from thermal conduction through the interstice, filled with theelectrically insulating solid, radially directly into the module wall,which for instance is of metal, enhances the cooling effect incomparison with thermal conduction exclusively to the actuator base.

Advantageously, gas or air bubbles are admixed with the material; theyprevent a major pressure development in the interior of the actuatormodule.

The viscosity of the material used as the cooling medium can be selectedeither in such a way that it does change to a fluid liquid state evenwhen heated, or alternatively in such a way that the material, such as awax, changes to the liquid state at higher temperatures, which can be inthe range between 50° C. and 100° C. The formation of the liquid phaseof the material at elevated temperature intensifies the cooling, and thecooling remains sufficiently strong.

At room temperature and below, the actuator exhibits a somewhat shorterstroke. The phase of the shorter stroke is quickly completed as a resultof faster heating.

In the solid state of the material, the cooling is only moderate; thatis, by intrinsic heating, the actuator changes very quickly over to theoperating state having the greater stroke, before the cooling thenensues by liquefication of the material.

Care is preferably taken to assure that the interstice of thepiezoelectric actuator of the invention is not completely filled withmaterial, so that when the actuator heats up, this material can expandinto a chamber that contains air or gas.

So that no hindrance of the actuator stroke will ensue, the module wall,which for instance is of metal, is coated on the inside with ananti-stick coating, which prevents the material from adhering to thewall.

The surface of the actuator body can also be provided with an anti-stickcoating of this kind, for instance of PTFE.

Also preferably, the actuator base is sealed off from the intersticethat contains the material by a sealing agent, such as an adhesive, sothat the liquefied material cannot escape from the actuator module.

Further characteristics and advantages of the invention will becomeclear below from the description of the ensuing exemplary embodiments ofthe invention taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic longitudinal section view of a first embodimentof the invention, with the section changing in the area of cut line 13,which is placed as it is to depict the electrical connection on theright hand side, and show the connection means for the spring on theleft side.

FIG. 2 shows a schematic longitudinal section view of a secondembodiment of a piezoelectric actuator of the invention, with thesection changing in the area of cut line 13, which is placed as it is todepict the electrical connection on the right hand side, and show theconnection means for the spring on the left side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first exemplary embodiment, shown in fragmentary longitudinalsection in FIG. 1, of a piezoelectric actuator of the invention, anactuator body 1, which can take the form of a multilayer laminate ofstacked layers of piezoelectric material and metal or electricallyconductive layers between them and acting as electrodes, with its lowerface end fixed on an actuator base 6 and surrounded by a module wall 2with an interstice 7 being maintained. A spring band 12 on the left andon the right elastically prestresses the actuator body 1 between theactuator base 6 and an upper ring flange (not identified by referencenumeral).

If the actuator body 1 is acted upon at its electrodes with a pulsatingelectrical voltage, it executes analogously pulsating strokes, changingthe spacing between its face ends fastened between the upper ring flangeand the actuator base 6 by the spring bands 12. These strokes aretransmitted to the needle (not shown) of a valve (see action arrow S)via a piston 10. It should also be noted that the electrode supply leadsextend toward the bottom to lead clamps not identified by referencenumeral.

In the embodiment of FIG. 1, the interstice 7 between the actuator body1 and the metal module wall 2 is filled with a low-viscosity elastomer3, or some other elastic solid, which is electrically insulating.Furthermore, as indicated in FIG. 1, heat-conducting particles 4 areadmixed with the elastic solid 3.

The solid 3 is elastic and has low viscosity, so as not to hinder thestrokes of the actuator core. To accommodate the different thermalexpansion of the solid 3 compared to the other components, gas or airbubbles 5 are also added to the solid, so that high pressures will notoccur when the actuator heats up. The gas or air bubbles can also beunited in collected form in a larger volume.

In the embodiment shown in FIG. 1, the nature of the solid 3, that is,the elastomer, is selected such that over the entire operatingtemperature range of the actuator, it does not yet change to a fluidliquid state. As a result, no special effort is needed for sealing offthe actuator base 6.

In FIG. 1, it should also be noted that the solid 3, optionally togetherwith the heat-conducting particles 4 and the air bubbles 5, also fillsthe space above the upper ring flange, that is, the space 11 around thepiston 10, since this space is not closed off from the lower part of theinterstice 7.

The embodiment shown in FIG. 2 of the piezoelectric actuator of theinvention improves the cooling still further by providing that theinterstice 7 is filled with a material 3, such as a wax, which changesover to the liquid state at elevated temperatures, for instance in therange from 50° C. to 100° C. Thus rapid heating of the actuator tooperating temperature and after that intensive cooling are attainedbecause of the liquid state of the material 3 acting as a heatconduction medium. Upon disassembly, there are no refilling problems,since in the cooled state the material 3 is in the solid state. Comparedwith the elastic heat-conducting solid 3 of FIG. 1, the material 3acting as the heat-conducting medium in the embodiment shown in FIG. 2is more plastic in nature, if a wax is selected to serve that purpose.

In the upper part, that is, the space 11 surrounding the piston 10, anair or gas filling is provided in FIG. 2, which in turn serves to absorbexpansion upon thermal expansion of the material 3.

To prevent the material in the solid state from hindering actuatorexpansion, the wall 2 of the actuator module is provided on the insidewith an anti-stick coating 9, which prevents the material 3 fromadhering to the inner wall. This anti-stick coating can for instance beof PTFE, such as Teflon™. The surface of the actuator body 1 can also beprovided with such an anti-stick coating.

The advantage is that at low temperatures (down to 40° C.), the coolantmedium 3 separates from the wall because of shrinkage, and the heatconductivity is thus worsened still further, causing the actuator 1 toreach its operating temperature faster. This characteristics alsoapplies to FIG. 1, with the filling of elastomer 3 and anti-stickcoating on the inner wall of 2.

It should also be noted that in the embodiment of the piezoelectricactuator shown in FIG. 2, the bottom plate 6, because of the liquidphase of the coolant 3, is sealed off by a sealing means 8, such asadhesive.

Thus in the exemplary embodiment shown in FIG. 2, the formation of aliquid phase of the solid 3 intensifies the cooling at elevatedtemperature, especially at operating temperature. At room temperatureand below, the actuator body 1 can have a shorter stroke. In the solidstate of the material 3, the cooling action by thermal conduction of thematerial 3 is reduced; that is, the actuator body 1 very rapidly changesby intrinsic heating into its operating state with a longer stroke,before the cooling then ensues by liquefication of the material 3.

The anti-stick coating of the module wall 2 and optionally of the wallof the actuator body 1 as well prevents any possible stroke reduction ofthe actuator from the material 3.

The foregoing relates to a preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

I claim:
 1. A piezoelectric actuator, for actuating control valves orinjection valves in internal combustion engines in motor vehicles,having a piezoelectric actuator body (1) formed of a multilayer laminateof layered plies of piezoelectric material and intervening electricallyconductive layers acting as electrodes, the actuator body (1) has a faceend fixed on an actuator base (6), and the actuator body (1) issurrounded by a module wall (2) defining an interstice therebetween (7),wherein the interstice (7) is filled with an electrically insulatingmaterial (3) having good thermal conductivity, which insulating materialis solid at least in a temperature range below an operating temperatureof the actuator and above −50° C., and wherein said electricallyinsulating material is a plastic like material, in which the solidmaterial (3) is filled with heat-conducting filler (4), and in which gasor air bubbles (5) are admixed with the solid material (3).
 2. Thepiezoelectric actuator of claim 1, in which said electrically conductinglayers are metal.
 3. The piezoelectric actuator of claim 1, in whichsaid electrically insulting material is elastic.
 4. The piezoelectricactuator of claim 1, in which a viscosity of the material (3) isselected such that even upon heating the material does not change overto a fluid liquid state.
 5. The piezoelectric actuator of claim 1, inwhich the heat-conducting material (3), is selected to change over tothe liquid state at elevated temperatures.
 6. A piezoelectric actuator,for actuating control valves or injection valves in internal combustionengines in motor vehicles, having a piezoelectric actuator body (1)formed of a multilayer laminate of layered plies of piezoelectricmaterial and intervening electrically conductive layers acting aselectrodes, the actuator body (1) has a face end fixed on an actuatorbase (6), and the actuator body (1) is surrounded by a module wall (2)defining an interstice therebetween (7), wherein the interstice (7) isfilled with an electrically insulating material (3) having good thermalconductivity, which material is solid at least in a temperature rangebelow an operating temperature of the actuator, and wherein the material(3) that fills the interstice (7) is a solid comprising a low-viscosityelastomer.
 7. The piezoelectric actuator of claim 6, in which that thesolid material (3) is filled with heat-conducting filler (4).
 8. Thepiezoelectric actuator of claim 6, in which gas or air bubbles (5) areadmixed with the solid material (3).
 9. A piezoelectric actuator, foractuating control valves or injection valves in internal combustionengines in motor vehicles, having a piezoelectric actuator body (1)formed of a multilayer laminate of layered plies of piezoelectricmaterial and intervening electrically conductive layers acting aselectrodes, the actuator body (1) has a face end fixed on an actuatorbase (6), and the actuator body (1) is surrounded by a module wall (2)defining an interstice therebetween (7), wherein the interstice (7) isfilled with an electrically insulating material (3) having good thermalconductivity, which material is solid at least in a temperature rangebelow an operating temperature of the actuator, and in which gas or airbubbles (5) are admixed with the solid material (3).
 10. A piezoelectricactuator, for actuating control valves or injection valves in internalcombustion engines in motor vehicles, having a piezoelectric actuatorbody (1) formed of a multilayer laminate of layered plies ofpiezoelectric material and intervening electrically conductive layersacting as electrodes, the actuator body (1) has a face end fixed on anactuator base (6), and the actuator body (1) is surrounded by a modulewall (2) defining an interstice therebetween (7), wherein the interstice(7) is filled with an electrically insulating material (3) having goodthermal conductivity, which material is solid at least in a temperaturerange below an operating temperature of the actuator, and material (3)is selected to change over to the liquid state at elevated temperatures,and wherein the interstice (7) is not completely filled by the material(3), so that the material on heating up can expand into a space (11)that contains air or gas.
 11. The piezoelectric actuator of claim 10, inwhich a liquefaction temperature of the material (3) is selected to bebetween 50° C. and 100° C.
 12. The piezoelectric actuator of claim 10,in which the material (3) is a wax.
 13. The piezoelectric actuator ofclaim 10, in which the module wall (2) is coated on an inside thereofwith an anti-stick coating (9), which coating prevents the material (3)from adhering to the wall.
 14. The piezoelectric actuator of claim 13,in which the anti-stick coating (9) comprises PTFE.
 15. Thepiezoelectric actuator of claim, 10, in which an external surface of theactuator body (1) is provided with an anti-stick coating (9).
 16. Thepiezoelectric actuator of claim 10, in which the actuator base (6) issealed off from the interstice (7) containing the material (3) by asealing means.
 17. The piezoelectric actuator of claim 16, in which thesealing means is an adhesive.